It has long been a goal to develop an engine that operates on thermal energy that is freely available in the ambient environment. Consistent with the second law of thermodynamics, prior attempts at such thermal-energy-harvesting devices required two distinct sources of thermal energy, namely, a heat source and a heat sink for supplying and removing heat, respectively, at different temperatures simultaneously. A heat-source and heat-sink pair having two distinct, spaced-apart temperatures typically does not occur naturally and/or plentifully, and thus are generally difficult to access. Therefore, because ambient heat at a single atmospheric temperature is more abundant and available than a simultaneous dual-heat source, a device for harnessing single-source ambient heat is more desirable than a device that requires a dual-heat source.
The present inventor disclosed a device in U.S. Pat. No. 6,899,967. That device relies on cyclic temperature changes in the environment to produce the needed simultaneous dual-heat source. The needed temperature difference was provided through the use of a mass of material that has significant heat capacity. The prior device is a thermo-electrochemical converter that operates on a pressure difference between two metal-hydride chambers separated by a membrane electrode assembly (MEA). In the prior invention, one metal-hydride chamber is exposed to the ambient environment while the other is insulated and thermally stabilized. A thermal mass is coupled to the stabilized chamber to act as a heat source/sink material. Insulation may be used to thermally isolate the thermal-mass material from the environment in order to enhance the temperature difference produced. It absorbs heat and stores it during periods of elevated ambient temperature and releases that heat to function as an elevated-temperature heat source during periods of reduced ambient temperatures. As such, changes in the temperature of the thermal mass will always lag temperature changes in its environment. Thus a converter coupled between the thermal mass and the environment will be subjected to a simultaneous temperature differential needed for the device to operate.
The open-circuit electrical potential due to a hydrogen pressure differential across a proton-conductive membrane electrode assembly (MEA) is a linear function of temperature and proportional to the natural logarithm of the hydrogen pressure ratio and can be calculated using the Nernst equation (Fuel Cell Handbook, Fourth Edition, 1999, by J. H. Hirschenhofer, D. B. Stauffer, R. R. Engleman, and M. G. Klett, at pp. 2-5:VOC= RT/2F ln(PHi/PLow)  Equation 1where VOC is open circuit voltage, R is the universal gas constant, T is the cell absolute temperature in degrees Kelvin, F is Faraday's constant, PHi is the hydrogen pressure on the high-pressure side and PLow is the hydrogen pressure on the low-pressure side.
The hydrogen pressure produced by a metal-hydride bed depends on temperature. When the ambient metal-hydride chamber is at a higher temperature, H2 gas is desorbed from its metal hydride content and flows through the MEA into the thermally stabilized chamber, thus generating power. During the next half cycle, when the temperature of the ambient chamber falls below the temperature of the insulated chamber, the opposite takes place, hydrogen flows through the MEA back to the ambient temperature chamber. Hydrogen thus cycles back and forth under a pressure differential across the proton-conductive membrane generating power in the process.
A major limitation encountered with the prior invention is associated with the need to have a device that is capable of scavenging power in a relatively efficient manner. A major limitation in achieving efficient operation is associated with the difficulty of creating a significant temperature difference between components. This is particularly true for a small device. The close proximity of the components in a small device allows parasitic heat transfer losses between the two metal-hydride beds that are too high whenever a significant temperature gradient is present. In larger devices, the need for insulation and heat sink/source material can result in a large, bulky device that is difficult to implement. Thus it can be appreciated that a need remains for a device for producing electrical power using heat from its ambient environment that overcomes the disadvantages and shortcomings of previous chemical and thermal converters that need a simultaneous temperature difference in order to operate.